Platinum surface coating and method for manufacturing the same

ABSTRACT

An improved platinum surface coating and method for manufacturing the improved platinum surface coating wherein the platinum surface coating having a fractal surface coating of platinum [“platinum gray”] with a increase in surface area of at least 5 times when compared to shiny platinum of the same geometry and also having improved resistance to physical stress when compared to platinum black having the same surface area. The process of electroplating the surface coating of platinum gray comprising plating at a moderate rate, i.e., at a rate that is faster than the rate necessary to produce shiny platinum and that is less than the rate necessary to produce platinum black.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No.: 10/226,976,filed Aug. 23, 2002, which claims the benefit of U.S. ProvisionalApplication No. 60/372,062, filed Apr. 11, 2002, the disclosure of whichis incorporated herein by reference.

FEDERALLY SPONSORED RESEARCH

This invention was made with government support under grant No.R24EY12893-01, awarded by the National Institutes of Health. TheGovernment has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the invention relates to platinum surface coating andelectroplating processes for deposition of platinum.

2. Description of Related Art

Platinum has often been used as a material for electrodes in corrosiveenvironments such as the human body due to its superior electricalcharacteristics, biocompatibility and stability. Platinum has manydesirable qualities for use as an electrode for electrical stimulationof body tissue. Since platinum has a smooth surface and its surface areais limited by the geometry of the electrode, it is not efficient fortransferring electrical charge. The platinum with a smooth surface ishereinafter called “shiny platinum”.

Electrodes for stimulating body tissue by electrical stimulation areknown in great variety. For the utility of an implantable stimulation orsensing electrode—especially one intended for long-term use in a tissuestimulator with a non-renewable energy source and that, therefore, mustrequire minimal energy—a high electrode capacitance and correspondinglylow electrical impedance is of great importance. Furthermore, withoutsufficiently low impedance, a large voltage may cause polarization ofboth the electrode and the tissue to which the electrode is attachedforming possibly harmful byproducts, degrading the electrode anddamaging the tissue.

Because the ability of an electrode to transfer current is proportionalto the surface area of the electrode and because small electrodes arenecessary to create a precise signal to stimulate a single nerve orsmall group of nerves, many in the art have attempted to improve theability of an electrode to transfer charge by increasing the surfacearea of the electrode without increasing the size of the electrode.

One approach to increase the surface area of a platinum electrodewithout increasing the electrode size and therefore to improve theability of the electrode to transfer charge is to electroplate platinumrapidly such that the platinum molecules do not have time to arrangeinto a smooth, shiny surface. The rapid electroplating forms a platinumsurface which is commonly known as “platinum black”. Platinum black hasa porous and rough surface which is less dense and less reflective thanshiny platinum. U.S. Pat. No. 4,240,878 to Carter describes a method ofplating platinum black on tantalum.

Platinum black is more porous and less dense than shiny platinum.Platinum black has weak structural and physical strength and istherefore not suitable for applications where the electrode is subjectto even minimal physical stresses. Platinum black also requiresadditives such as lead to promote rapid plating. Lead, however, is aneurotoxin and cannot be used in biological systems. Finally, due toplatinum black's weak structure, the plating thickness is quite limited.Thick layers of platinum black simply fall apart.

For the foregoing reasons there is a need for an improved platinumsurface coating and process for electroplating the surface to obtain anincreased surface area for a given geometry and at the same time thecoating is structurally strong enough to be used in applications wherethe platinum surface coating is subject to physical stresses.

BRIEF SUMMARY OF INVENTION

The present invention is directed in part to a platinum surface coatinghaving increased surface area for greater ability to transfer charge andalso having sufficient physical and structural strength to withstandphysical stress.

This and other aspects of the present invention which may become obviousto those skilled in the art through the following description of theinvention are achieved by an improved platinum surface coating andmethod for preparing the improved platinum surface coating having afractal surface coating of platinum, hereinafter called “platinum gray”.Platinum gray has an increase in surface area of at least 5 timescompared to shiny platinum of the same geometry. Platinum gray has atthe same time an improved resistance to physical stress when compared toplatinum black. The gray color is not considered a feature of theinvention. It is a means of describing the invention. The process ofelectroplating the surface coating of platinum gray comprising platingat a moderate rate, i.e., at a rate that is faster than the ratenecessary to produce shiny platinum and that is less than the ratenecessary to produce platinum black.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a platinum gray surface magnified 2000 times.

FIG. 2 shows a shiny platinum surface magnified 2000 times.

FIG. 3 shows a platinum black surface magnified 2000 times.

FIG. 4 shows color density (D) values and lightness (l*) values forseveral representative samples of platinum gray, platinum black andshiny platinum.

FIG. 5 shows a three-electrode electroplating cell with a magneticstirrer.

FIG. 6 shows a three-electrode electroplating cell in an ultrasonictank.

FIG. 7 shows a three-electrode electroplating cell with a gas dispersiontube.

FIG. 8 shows an electroplating system with constant voltage control orconstant current control.

FIG. 9 shows an electroplating system with pulsed current control.

FIG. 10 shows an electroplating system with pulsed voltage control.

FIG. 11 shows an electroplating system with scanned voltage control.

FIG. 12 shows an electrode platinum silicone array having 16 electrodes.

FIG. 13 shows the electrode capacitance for both plated and unplatedelectrodes of varying diameter.

FIG. 14 shows a representative linear voltage sweep of a representativeplatinum electrode.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, an illustrative example of a platinum gray surfacecoating for an electrode is shown having a fractal surface with asurface area increase of greater than 5 times the surface area for ashiny platinum surface of the same geometry, shown in FIG. 2, and anincrease in strength over a platinum black surface, shown in FIG. 3.FIGS. 1, 2, and 3 are images produced on a Scanning Electron Microscope(SEM) at 2000× magnifications taken by a JEOL JSM5910 microscope (Tokyo,Japan). Under this magnification level it is observed that platinum grayis of a fractal configuration having a cauliflower shape with particlesizes ranging from 0.5 to 15 microns. Each branch of such structure isfurther covered by smaller and smaller particles of similar shape. Thesmallest particles on the surface layer may be in the nanometer range.This rough and porous fractal structure increases the electrochemicallyactive surface area of the platinum surface when compared to anelectrode with a smooth platinum surface having the same geometricshape.

The surface is pure platinum because no impurities or other additivessuch as lead need to be introduced during the plating process to produceplatinum gray. This is especially advantages in the field of implantableelectrodes because lead is neurotoxin and cannot be used in the processof preparing implantable electrodes. Alternatively, other materials suchas iridium, rhodium, gold, tantalum, titanium or niobium could beintroduced during the plating process if so desired but these materialsare not necessary to the formation of platinum gray.

Platinum gray can also be distinguished from platinum black and shinyplatinum by measuring the color of the material on a spectrodensitometerusing the Commission on Illumination l*a*b* color scale. l* defineslightness, a* denotes the red/green value and b*, the yellow/blue value.The lightness value (called l* Value) can range from 0 to 100, wherewhite is 100 and black is 0—similar to grayscale. The a* value can rangefrom +60 for red and −60 for green, and the b* value can range from +60for yellow and −60 for blue. All samples measured have very small a* andb* values (they are colorless or in the so called neutral gray zone),which suggests that the lightness value can be used as grayscale forPlatinum coatings.

Referring to FIG. 4, the l*, a*, and b* values for representativesamples of platinum gray, platinum black and shiny platinum are shown asmeasured on a color reflection spectrodensimeter, X-Rite 520. Platinumgray's l* value ranges from 25 to 90, while platinum black and shinyplatinum both have l* values less than 25.

Referring to FIG. 4, color densities have also been measured forrepresentative samples of platinum gray, platinum black and shinyplatinum. Platinum gray's color density values range from 0.4 D to 1.3D; while platinum black and shiny platinum both have color densityvalues greater than 1.3 D.

Platinum gray can also be distinguished from platinum black based on theadhesive and strength properties of the thin film coating of thematerials. Adhesion properties of thin film coatings of platinum grayand platinum black on 500 microns in diameter electrodes have beenmeasured on a Micro-Scratch Tester (CSEM Instruments, Switzerland). Acontrolled micro-scratch is generated by drawing a spherical diamond tipof radius 10 microns across the coating surface under a progressive loadfrom 1 millinewton to 100 millinewtons with a 400 micron scratch length.At a critical load the coating will start to fail. Using this test it isfound that platinum gray can have a critical load of over 60millinewtons while platinum black has a critical load of less than 35millinewtons.

Referring to FIGS. 5, 6, 7 and 8, a method to produce platinum grayaccording to the present invention is described comprising connecting aplatinum electrode 2, the anode, and a conductive substrate to be plated4, the cathode, to a power source 6 with a means of controlling andmonitoring 8 either the current or voltage of the power source 6. Theanode 2, cathode 4, a reference electrode 10 for use as a reference incontrolling the power source 6 and an electroplating solution are placedin a electroplating cell 12 having a means 14 for mixing or agitatingthe electroplating solution. Power is supplied to the electrodes withconstant voltage, constant current, pulsed voltage, scanned voltage orpulsed current to drive the electroplating process. The power source 6is modified such that the rate of deposition will cause the platinum todeposit as platinum gray, the rate being greater than the depositionrate necessary to form shiny platinum and less than the deposition ratenecessary to form platinum black.

Referring to FIGS. 5, 6 and 7, the electroplating cell 12, is preferablya 50 ml to 150 ml four neck glass flask or beaker, the common electrode2, or anode, is preferably a large surface area platinum wire orplatinum sheet, the reference electrode 10 is preferably a Ag/AgClelectrode (silver, silver chloride electrode), the conductive substrateto be plated 4, or cathode, can be any suitable material depending onthe application and can be readily chosen by one skilled in the art.Preferable examples of the conductive substrate to be plated 4 includebut are not limited to platinum, iridium, rhodium, gold, tantalum,titanium or niobium.

The stirring mechanism is preferably a magnetic stirrer 14 as shown inFIG. 5, an ultrasonic tank 16 (such as the VWR Aquasonic 50D) as shownin FIG. 6, or gas dispersion 18 with Argon or Nitrogen gas as shown inFIG. 7. The plating solution is preferably 3 to 30 mM (milimole)ammonium hexachloroplatinate in disodium hydrogen phosphate, but may bederived from any chloroplatinic acid or bromoplatinic acid or otherelectroplating solution. The preferable plating temperature isapproximately 24 to 26° C.

Electroplating systems with pulsed current and pulsed voltage controlare shown in FIGS. 9 and 10 respectively. While constant voltage,constant current, pulsed voltage or pulsed current can be used tocontrol the electroplating process, constant voltage control of theplating process has been found to be most preferable. The mostpreferable voltage range to produce platinum gray has been found to be−0.45 Volts to −0.85 Volts. Applying voltage in this range with theabove solution yields a plating rate in the range of about 1 micron perminute to 0.05 microns per minute, the preferred range for the platingrate of platinum gray. Constant voltage control also allows an array ofelectrodes in parallel to be plated simultaneously achieving a fairlyuniform surface layer thickness for each electrode.

The optimal potential ranges for platinum gray plating are solution andcondition dependent. Linear voltage sweep can be used to determine theoptimal potential ranges for a specific plating system. A representativelinear voltage sweep is shown in FIG. 14. During linear voltage sweep,the voltage of an electrode is scanned cathodically until hydrogen gasevolution occurs which reveals plating rate control steps of electrontransfer 20 and diffusion 22. For a given plating system, it ispreferable to adjust the electrode potential such that the platinumreduction reaction has a limiting current under diffusion control ormixed control 24 between diffusion and electron transfer but that doesnot result in hydrogen evolution 26.

It has been found that because of the physical strength of platinumgray, surface layers of thickness greater than 30 microns can be plated.It is very difficult to plate shiny platinum in layers greater thanapproximately several microns because the internal stress of the denseplatinum layer which will cause the plated layer to peel off and theunderlying layers cannot support the above material. The additionalthickness of the plate's surface layer allows the electrode to have amuch longer usable life.

The following example is illustrative of electroplating platinum on aconductive substrate to form a surface coating of platinum gray.

Electrodes with a surface layer of platinum gray are prepared in thefollowing manner using constant voltage plating. An electrode platinumsilicone array having 16 electrodes where the diameter of the platinumdiscs on the array range from 510 to 530 microns, as shown in FIG. 12,is first cleaned electrochemically in sulfuric acid and the startingelectrode impedance is measured in phosphate buffered saline solution.Referring to FIG. 5, the electrodes are arranged in the electroplatingcell such that the plating electrode 2 is in parallel with the commonelectrode 4. The reference electrode 10 is positioned next to theelectrode 4. The plating solution is added to the electroplating cell 12and the stirring mechanism 14 is activated.

A constant voltage is applied on the plating electrode 2 as compared tothe reference electrode 10 using an EG&G PAR M273 potentiostat 6. Theresponse current of the plating electrode 2 is recorded by a recordingmeans 8. (The response current is measured by the M273 potentiostat 6.)After a specified time, preferably 1-90 minutes, and most preferably 30minutes, the voltage is terminated and the electrode 4 is thoroughlyrinsed in deionized water.

The electrochemical impedance of the electrode array with the surfacecoating of platinum gray is measured in a saline solution. Thecharge/charge density and average plating current/current density arecalculated by integrating the area under the plating current vs. timecurve. Scanning Electron Microscope (SEM)/Energy Dispersed Analysis byX-ray (EDAX™) analysis can be performed on selected electrodes. SEMMicrographs of the plated surface can be taken showing its fractalsurface. Energy Dispersed Analysis demonstrates that the sample is pureplatinum rather than platinum oxide or some other materials.

From this example it is observed that the voltage range is mostdeterminative of the formation of the fractal surface of platinum gray.For this system it observed that the optimal voltage drop across theelectrodes to produce platinum gray is approximately −0.55 to −0.65Volts vs. Ag/AgCl reference electrode 10. The optimal platinumconcentration for the plating solution is observed to be approximately 8to 18 mM ammonium hexachloroplatinate in 0.4 M (Mole) disodium hydrogenphosphate.

FIG. 12 provides a perspective view of a retinal electrode array for usewith the present invention, generally designated 32, comprisingoval-shaped electrode array body 34, a plurality of electrodes 36 madeof a conductive material, such as platinum or one of its alloys, butthat can be made of any conductive biocompatible material such asiridium, iridium oxide or titanium nitride, and a single referenceelectrode 38 made of the same material as electrode 36, wherein theelectrodes are individually attached to separate conductors 40 made of aconductive material, such as platinum or one of its alloys, but whichcould be made of any biocompatible conductive material, that isenveloped within an insulating sheath 42, that is preferably silicone,that carries an electrical signal to each of the electrodes 36.

A strain relief internal tab 44, defined by a strain relief slot 46 thatpasses through the array body 34, contains a mounting aperture 48 forfixation of the electrode array body 34 to the retina of the eye orother neural interface by use of a surgical tack. A reinforcing ring 50is colored and opaque to facilitate locating the mounting aperture 48during surgery. A grasping handle 52 is located on the surface ofelectrode array body 34 to enable its placement by a surgeon usingforceps or by placing a surgical tool into the hole formed by graspinghandle 52. Grasping handle 52 avoids damage to the electrode body thatmight be caused by the surgeon grasping the electrode body directly. Theelectrode array 32 is described in greater detail in U.S. patentapplication No. 2002/0111658 A1 filed Feb. 13, 2001 and entitledImplantable Retinal Electrode Array Configuration for Minimal RetinalDamage and Method of Reducing Retinal Stress, which is incorporatedherein by reference.

FIG. 13 shows the increase in electrode capacitance of severalelectrodes of varying diameter for a polyimide array plated according tothe above example at −0.6 V vs. Ag/AgCl Reference electrode for 30minutes compared with unplated electrodes of the same diameters. Becausethe electrode capacitance is proportional to its surface area, thesurface area increase, calculated from electrode capacitance, is 60 to100 times that of shiny platinum for this array. It should be noted thatshiny platinum exhibits some roughness and has a surface area increaseup to 3 times that of the basic geometric shape. While it is simple tomeasure a surface area change between two sample using capacitance, itis difficult to compare a sample with the basic geometric shape.

As plating conditions, including but not limited to the platingsolution, surface area of the electrodes, pH, platinum concentration andthe presence of additives, are changed the optimal controlling voltageand/or other controlling parameters will also change according basicelectroplating principles. Platinum gray will still be formed so long asthe rate of deposition of the platinum particles is slower than that forthe formation of platinum black and faster than that for the formationof shiny platinum.

While the invention is described in terms of a specific embodiment,other embodiments could readily be adapted by one skilled in the art.Accordingly, the scope of the invention is limited only by the followingclaims.

1. A platinum surface coating, comprising: a conductive substrate; and asurface coating of platinum having a fractal configuration.
 2. Theplatinum surface coating of claim 1 wherein said surface coating has atleast 5 times the surface area of that for the corresponding surfacearea resulting from the basic geometric shape.
 3. The platinum surfacecoating of claim 1 wherein said surface coating has a surface area ofless than 500 times the corresponding surface area resulting from thebasic geometric shape.
 4. The platinum surface coating of claim 1wherein said surface coating has a surface area of less than 200 timesthe corresponding surface are resulting from the basic geometric shape.5. The platinum surface coating of claim 1 wherein said surface coatinghas a thickness of at least 0.5 microns.
 6. The platinum surface coatingof claim 1 wherein said surface coating has a thickness of at least 5microns.
 7. The platinum surface coating of claim 1 wherein said surfacecoating has a thickness of at least 10 microns.
 8. The platinum surfacecoating of claim 1 wherein said surface coating has a thickness of atleast 30 microns.
 9. The platinum surface coating of claim 1 whereinsaid surface coating has an adhesive strength as measured by criticalload greater than 35 millinewtons.
 10. The platinum surface coating ofclaim 1 wherein said surface coating appears gray in color.
 11. Theplatinum surface coating of claim 1 wherein said surface coating has alightness (l*) greater than 30 on the CIELAB color scale.
 12. Theplatinum surface coating of claim 1 wherein said surface coating has acolor density (D) greater than 0.25 D but less than 1.3 D.
 13. Theplatinum surface coating of claim 1 wherein said surface coating ofplatinum comprises alloys of platinum and iridium or rhodium.
 14. Theplatinum surface coating of claim 1 wherein said surface coating ofplatinum comprises alloys of platinum and gold, tantalum, titanium orniobium.
 15. The platinum surface coating of claim 1 wherein saidconductive substrate is platinum, platinum alloy, iridium, iridium oxideor rhodium.
 16. The platinum surface coating of claim 1 wherein saidconductive substrate is gold, tantalum, titanium, titanium nitride orniobium.
 17. A method for electroplating a platinum surface coatinghaving a rough surface, comprising: electroplating the surface of aconductive substrate at a rate such that the particles of platinum formon the conductive substrate faster than necessary to form shiny platinumand slower than necessary to form platinum black.
 18. The method ofclaim 17 wherein at least a portion of said rough surface coating hasfractal geometry.
 19. The method of claim 17 wherein said step ofelectroplating is accomplished at a rate of more than 0.05 microns perminute, but less than 1 micron per minute.
 20. The method of claim 17wherein the electroplating process is controlled by electrode voltage.21. The method of claim 20 wherein the voltage is constant voltage. 22.The method of claim 20 wherein the controlled voltage causes at least apartially diffusion-limited plating reaction.
 23. The method of claim 17wherein the electroplating is accomplished at a rate of more than 0.05microns per minute, but less than 1 micron per minute.
 24. The method ofclaim 17 wherein the voltage of the electroplating process is less than0.2 Volts and greater than −1 Volts vs. Ag/AgCl Reference electrode. 25.The method of claim 17 wherein the voltage of the electroplating processis less than −0.45 Volts and greater than −0.85 Volts vs. Ag/AgClReference electrode.
 26. The method of claim 17 wherein theelectroplating solution is at least 3 mM but less than 30 mM ammoniumhexachloroplatinate in about 0.4 M disodium hydrogen phosphate.
 27. Aplatinum surface coating prepared by the method of claim 17.